The ring spring, such as a disk spring and a wave spring, has been used for absorbing shocks that occur during engaging a clutch in a clutch device in transportation equipment, for example. In a conventional method for producing the ring spring, a blank having approximately a ring shape has been punched out of a tabular material by pressing processing; however, in this method, the yield ratio of the material was poor. As a method to improve the yield ratio of the material, a method is known in which edge parts of material formed in a ring shape by bending formation are welded (see Japanese Unexamined Patent Application Publications Nos. Hei 06 (1994)-106277 and 2001-225112).
In a method for producing disk spring, in the case in which edge parts of the material are welded, a process shown in FIG. 4 is employed, for example. First, material is prepared (step S11), the material is formed in approximately a ring shape by bending formation (step S12), and the edge parts of the approximately ring shape material are mutually welded (step S13). According to this method, ring material having no edge can be obtained. Next, the ring material is processed to a disk shape (approximately cone shape) by cold forming (disk forming) so as to obtain disk spring material (step S14). Subsequently, by performing quenching and tempering of the disk spring material (steps S15 and S16) so as to obtain the disk spring.
However, in the method in which the edge parts of the raw material are welded, the welded part of the material becomes a welded metal part, and a circumferential part thereof becomes a welded heat-affected zone that is affected by heating during welding. In this case, since a hardened region is formed at the welded metal part and a softened region is formed at the welded heat-affected zone, in order to keep strength necessary as a disk spring, it is necessary that quenching and tempering be performed on the disk spring material after cold forming. Thus, since quenching and tempering are required and thereby increase the number of processes, production cost may be increased and the product may be expensive.
Therefore, an object of the present invention is to provide an inexpensive ring spring having high strength and a method for producing the same.